In an electrophotographic image forming apparatus, a two component developer is commonly used at the time of performing development. The two component developer includes a carrier. The carrier has a surface of a core portion covered with a resin or the like. In the core portion, a fine particle magnetic powder of ferrite is generally used. In a coating portion covering the core portion, a conductive agent such as carbon black is generally added for suppressing an increase in electrical resistivity of the carrier. By adding the conductive agent, the electrical resistivity of the carrier is adjusted to be in a suitable range. Thus, an electrification amount of a toner, which is electrified by friction between the toner and the carrier, is controlled to be in a suitable range. Accordingly, in image forming, a decrease in image density is prevented.
Recently, a technology is in practical use in which a developer containing a decoloring toner is used. According to such technology, printing with respect to paper and erasing of a printed portion are repeated, and thus the paper is reused. The decoloring toner is a toner of which a color is erased by heating at an arbitrary temperature.
When the printing is performed with respect to the paper by the decoloring toner, fixing is performed at a temperature lower (TL) than a decoloring temperature (TH). The printed portion formed by the decoloring toner is heated at a temperature greater than or equal to the decoloring temperature (TH), and thus the color of the toner is erased and becomes invisible.
When an image formed on the paper by such a decoloring toner is heated, it is preferable that the toner is made invisible by the heating, and thus the image on the paper cannot be seen by the naked eye.
However, when the two component developer is used, carbon black (in the coating portion of the carrier) or iron oxide (on the surface of the core portion of the carrier) may be adsorbed onto a photoreceptor along with the decoloring toner. In this case, the carbon black or the iron oxide is also attached to the paper along with the decoloring toner. For this reason, even when the paper is heated, the color of the carbon black or the iron oxide appears on the paper, so that a residual image of the erased color occurs. Thus the paper may not be reused.
When the conductive agent is not added to the coating portion of the carrier, the residual image of the erased color rarely occurs. However, when the conductive agent is not added to the coating portion, the electrification amount of the toner is excessively increased by an operation in which the developer is stirred or the like, and thus the development may not be performed.